Amide-based electrolyte battery

ABSTRACT

An energy storage apparatus is described and claimed herein comprising, generally, a battery housing enclosing a negative electrode, a positive electrode, and an electrolyte, wherein the electrolyte comprises a salt dissolved in either an amide-based solvent. In various embodiments, the amide-based solvent is a tertiary amide. Moreover, the energy storage apparatus may be a lithium ion battery that comprises an electrolyte with a lithium salt dissolved in a tertiary amide.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/966,959, having the title “AMIDE-BASED ELECTROLYTE BATTERY,” filed onApr. 30, 2018, the disclosure of which is incorporated herein byreference in its entirety. This application also claims priority to U.S.Provisional Application No. 62/492,661, titled “Amide-Based ElectrolyteBattery,” filed on May 1, 2017, the disclosure of which is furtherincorporated herein by reference in its entirety.

DESCRIPTION OF THE DRAWINGS

The drawings constitute a part of this specification and includeexemplary embodiments and examples of the Amide-Based ElectrolyteBattery, which may be embodied in various forms. It is to be understoodthat in some instances, various aspects of the invention may be shownexaggerated or enlarged to facilitate an understanding of the invention.Therefore the drawings may not be to scale.

FIG. 1 shows the generic structure of a tertiary amide wherein theR-groups may be the same or different. Additionally, depending on theembodiment, two or more R-groups may or may not connect to form a cyclicstructure.

FIG. 2 shows the structure and space filled model of severalelectrolytes that can be employed in a lithium battery, showing thefollowing molecules: (a) ethylene carbonate, (b)1,3-dimethyl-2-imidazolidinone, (c) dimethyl carbonate, (d)tetramethylurea, and (e) lithium bis(trifluoromethane) sulfonimide.

FIG. 3 is a graph showing Concentration dependent FTIR spectra forLiTFSI in (top) DMI and (bottom) TMU. Absorbance spectra were normalizedto the sum of the carbonyl stretch region.

FIG. 4 shows fractional area of the Li⁺-coordinated carbonyl stretch asa function of Li⁺ concentration in each of DMI and TMU.

FIG. 5 shows the FTIR-ATR spectra of the P-F stretch modes for LiPF₆ inTMU and LiPF₆ in dimethyl carbonate.

FIG. 6 shows the cyclic voltammagram of 1M LiTFSI in DMI and 1M LiTFSIin TMU.

FIG. 7 shows the room temperature conductivity of the amide-basedelectrolytes.

FIG. 8 shows the room temperature viscosity of the amide-basedelectrolytes.

TECHNICAL FIELD

The instant invention is directed to a means of storing energy. Theinvention relates generally to a rechargeable lithium ion battery. Moreparticularly, the instant invention is directed to a lithium batterythat employs an amide electrolyte for additional safety benefits overthe prior art.

Disclosure of the Invention BACKGROUND

As energy demands continue to expand, finding safe and efficient meansof storing energy is becoming a higher priority. Lithium ion batteriescurrently serve most portable energy storage needs, but pose significantsafety risks and efficiency issues.

Conventional lithium ion batteries consist of a positive electrode (e.g.LiCoO₂ with aluminum foil), a negative electrode (e.g. graphite withcopper foil), and an electrolyte medium. Current commercial lithium ionenergy storage technologies utilize electrolyte which are composed of alithium salt (usually lithium hexafluorophosphate (LiPF₆) or lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI)) and a combination oforganic carbonate solvents (usually a mixture of a linear carbonate anda cyclic carbonate in a 7:3 ratio by volume). Linear carbonates includebut are not limited to: dimethyl carbonate, ethyl methyl carbonate,diethyl carbonate, and their fluorinated derivatives. Cyclic carbonatesinclude but are not limited to: ethylene carbonate, propylene carbonate,butylene carbonate, and their fluorinated derivatives.

As previously indicated, conventional lithium ion batteries pose severesafety hazards, particularly due to the use of carbonate-basedelectrolytes. Organic carbonate solvents are proficient at maintaining acharge due to their intrinsically high dielectric constants. This hasled to their widespread usage in lithium ion batteries. However, theorganic carbonate solvents used are also highly flammable and exhibitlow heat tolerance. For instance, the flash points of some organiccarbonates commonly used in lithium ion battery electrolytes are aslisted: dimethyl carbonate 16° C., ethyl methyl carbonate 23° C.,diethyl carbonate 25° C., ethylene carbonate 143° C., propylenecarbonate 116° C., and butylene carbonate 135° C. The linear organiccarbonates, which compose 70% of the solvent mixture by volume haveflash points at or below room temperature, making lithium ion batterieshazardous, particularly when dropped, damaged, or operated at elevatedtemperatures. Aside from the obvious hazards that can arise, theseattributes also decrease the efficiency of lithium ion batteries as theyare not capable of operating safely over a wide temperature range.

The instant invention seeks to solve both of these dilemmas by creatinga novel lithium ion battery wherein the organic carbonate solvents arereplaced with tertiary amide solvents. It has been found that doing soprovides a solvent that has competitive dielectric constants, but with asignificant increase in flash point, thereby providing a safer batterythat can be employed across a much larger temperature gradient.

SUMMARY

The disclosed invention presents an novel lithium ion battery thatutilizes tertiary amide solvents to increase the safety of the batterywithout sacrificing efficacy. In one embodiment, the Amide-BasedElectrolyte Battery comprises a battery housing with a negativeelectrode, a positive electrode, and an electrolyte, for which theelectrolyte comprises a salt dissolved in either a single tertiary amidesolvent, a mixture of multiple tertiary amide solvents, or a mixture ofone or more tertiary amide solvents with one or more organic carbonatesolvents.

In additional embodiments, the battery is a lithium ion battery and thepositive electrode comprises lithium.

In alternate embodiments, the battery is a lithium ion battery and thesalt comprises lithium.

In additional embodiments, the battery is a lithium ion battery and atleast one of the positive electrode or the salt comprises lithium.

In alternate embodiments, the amide selected has a higher flash pointthan organic carbonates.

In alternate embodiments, the solvent comprises a tertiary amide.

In related embodiments, the solvent can comprise a tertiary amide thatcan be linear, cyclical, or comprise a combination of linear andcyclical tertiary amides.

In yet additional, related embodiments, the solvent comprises at leastone tertiary amide selected from the group comprising amide N-methylurea; N,N′-dimethyl urea; 1,1-dimethyl urea; Tetramethyl urea; N-ethylurea; 1,3-diethyl urea; 1,1-diethyl urea;N-[(trimethylsilyl)methyl]urea; 1-[3-(trimethoxysilyl)propyl]urea;1,3-dimethyl-2-imidazolidinone; 2-imidazolidinone;1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone;1,3-dihydro-2H-imidazol-2-one; 1-isopropyl-2-imidazolidinone; and1-methyl-2-imidazolidinone.

DETAILED DESCRIPTION

The subject matter of the present invention is described withspecificity herein to meet statutory requirements. However, thedescription itself is not intended to necessarily limit the scope ofclaims. Rather, the claimed subject matter might be embodied in otherways to include different steps or combinations of steps similar to theones described in this document, in conjunction with other present orfuture technologies. Although the terms “step” and/or “block” or“module” etc. might be used herein to connote different components ofmethods or systems employed, the terms should not be interpreted asimplying any particular order among or between various steps hereindisclosed unless and except when the order of individual steps isexplicitly described.

Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided, such asexamples of amides, imides, positive electrodes, and negativeelectrodes. For example, an exemplary embodiment of a battery isdiscussed herein to be a lithium ion battery due to its familiarity inthe industry. One skilled in the relevant art will recognize, however,that an Amide-Based Electrolyte Battery may be practiced without one ormore of the specific details, or with other methods, components,materials, and so forth. In other instances, well-known structures,materials, or operations are not shown or described in detail to avoidobscuring aspects of the invention.

A battery means is provided herein comprising, generally, a batteryhousing, a positive electrode, a negative electrode, and an amide-basedelectrolyte medium, wherein the positive electrode and negativeelectrode are housed within the housing, but spatially separated by theelectrolyte medium.

For the purposes of describing the instant invention, a comparison isdrawn to the conventional lithium ion battery. It is readily understoodthat this comparative embodiment is provided for illustrative purposesand is in no way intended to be limiting. To the contrary, it is readilyappreciated that the use of an amide-based electrolyte can be employedwith numerous types of batteries that employ a positive electrode andnegative electrode that must be bridged by an electrolyte.

As previously indicated, Lithium ion batteries consist of a positiveelectrode (e.g. LiCoO₂ with aluminum foil), a negative electrode (e.g.graphite with copper foil), and an electrolyte medium. The electrolytemedium is typically comprised of a lithium salt and one or more organiccarbonate solvents. The organic carbonates, particularly those of cyclicstructure, have particularly high dielectric constants, with ethylenecarbonate (EC) having a dielectric constant of 95.3 and dimethylcarbonate (DMC) having a dielectric constant of 3.08, both at 25° C.,leading to the historically widespread use of carbonate solvents inlithium ion battery electrolytes.

The present invention replaces the organic carbonate solvents in lithiumion battery electrolytes with amide-based electrolytes such as tertiaryamide solvents. It has been found that doing so increases the safetemperature operation spectrum without sacrificing the conductivity toan unacceptable level. It is conceivable that numerous amide-basedelectrolytes can be employed. Notable commercially available tertiaryamides include N-methyl urea (melting point 96° C., flash point notavailable); N,N′-dimethyl urea (157° C.); 1,1-dimethyl urea (meltingpoint 178° C., flash point not available); Tetramethyl urea (75° C.),i.e., TMU; N-ethyl urea (93° C.); 1,3-diethyl urea (melting point 112°C., flash point not available); 1,1-diethyl urea (melting point 69° C.,flash point not available); N-[(trimethylsilyl)methyl] urea (meltingpoint 115° C., flash point not available);1-[3-(trimethoxysilyl)propyl]urea (99° C.);1,3-dimethyl-2-imidazolidinone (95° C.); 2-imidazolidinone (meltingpoint 129° C., flash point not available);1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (120 ° C.);1,3-dihydro-2H-imidazol-2-one (not available);1-isopropyl-2-imidazolidinone (not available); and,1-methyl-2-imidazolidinone (not available).

Replacing the ethereal oxygens of the carbonates with amide groups doesnot result in large loss of dielectric constant, with1,3-dimethyl-2-imidazolidinone (DMI) having a dielectric constant of37.6 at 25° C. and tetramethylurea (TMU) having a dielectric constant of24.45 at 20° C. However, from a safety perspective, the amidederivatives have a significant increase in flash point and thus anincrease in safety. The typical commercially used lithium ion batteryelectrolyte has a flash point of ˜28° C. while DMI has a flash point of120° C. and TMU has a flash point of ˜75° C.

Tertiary amides exhibit dielectric constants similar to carbonates,while evidencing more favorable flashpoints. In some ways, thestructures share similarities to carbonates; yet, their differences arenotable and would suggest that tertiary amides would not be a goodsubstitute. Turning to FIG. 1, Tetramethylurea (TMU) and DMI, thetertiary amide most analogous of DMC and EC respectively, are shown. Thespace-filled models show that the carbonyl oxygens of the carbonates aremuch less crowded and thus more available to bind with the lithium ion(Li⁺), whereas in the tertiary amides the additional methyl groups crowdthe oxygen potentially making it less available to bind with Li⁺.Furthermore, the methyl groups of dimethyl carbonate (DMC) have theability to rotate, since the cis/trans energy barrier has been reportedto be small. However, in the linear tertiary amide, TMU, each nitrogenhas two methyl groups, inhibiting rotation and making the linear amidemuch more rigid than the linear carbonate. Thus, the change in structureof the solvent from carbonates to amides would be expected to havestructural, and potentially dynamical, effects on the electrolytesolution. Nevertheless, experiments have demonstrated that amide-basedelectrolytes can serve as a useful alternative to carbonate catalysts inbatteries, providing similar functionality with greater versatility.

EXAMPLE

Electrolytes formed from lithium bis(trifluromethane) sulfonamide(LiTFSI) in DMI and TMU were characterized using cyclic voltammetry,conductivity, viscosity, and Fourier transform infrared spectroscopy(FTIR) measurements.

EXPERIMENTAL METHODS

The following experimental methods were employed:

Solution Preparation: LiTFSI 98% pure from Acros Organics, LiPF₆ 99%pure from Acros Organics, TMU 99% pure from Alfa Aesar, and DMI 98% purefrom Alfa Aesar were used without further purification. The carbonylstretch region of the infrared spectra of both DMI and TMU were verifiedto be free of Fermi resonances. Electrolyte solutions in each of DMI andTMU were prepared with LiPF₆ and LiTFSI at varying concentrations.

Sample cells for FTIR were comprised of <5 μL of electrolyte between twoCaF₂ windows without spacer. All solutions and sample cells wereprepared in a N₂-filled glovebox in order to prevent watercontamination.

Fourier Transform Infrared Spectroscopy (FTIR): FTIR experiments wereperformed on a Bruker Tensor 27 equipped with a liquid nitrogen cooledMCT detector resulting in a spectral resolution of 0.5 cm⁻¹. Reportedspectra were averaged over 40 scans. FTIR were also collected inAttenuated Total Reflectance mode (ATR-FTIR) using a Bruker Tensor 27equipped with a Pike diamond/ZnSe ATR crystal and DTGS detector with aspectral resolution of 4 cm⁻¹. ATR-FTIR were averaged over 16 scans.FTIR data were modeled with Voigt profiles within the OrginLab software.

Cyclic Voltammetry: Cyclic voltammetry experiments were performed in anN2-filled glovebox using a three-electrode setup comprising a glassycarbon working electrode, a platinum counter electrode, and a silver(Ag/Ag⁺) reference electrode. The reference electrode was calibratedwith a ferrocene standard after each experiment.

Conductivity: Conductivity measurements were performed with a YCI 3200series conductivity instrument with a 3253 conductivity cell. In orderto minimize water contamination during measurements, the hydratedelectrodes of the cell were dried under flowing nitrogen gas to removewater not directly absorbed to the electrodes. Between measurements, theprobe was rinsed with ethanol followed by water to remove theelectrolyte solution and then soaked in water to maintain properhydration of the conductivity cell. A 0.1 M aqueous NaCl solution wasused to check the calibration of the conductivity probe.

Viscosity: Room temperature dynamic viscosity measurements werecollected on a Brookfield LVDV-II+Pro Cone/Plate viscometer equippedwith a CPE-40 cone spindle.

RESULTS

In order to study the structure of the Li+ solvation shell,concentration dependent FTIR spectra were collected for each of theamide-based electrolytes. Turning to FIG. 3, the FTIR spectra of theamide-based electrolytes show two bands in the carbonyl stretch region.Before the lithium salt is added, there is only one band in the carbonylstretch region of the infrared spectrum; this band is denoted as thefree band. The free carbonyl stretch band of TMU is excitonic due toordering of the neat TMU solvent. The low frequency C═O stretch bandgrows with increasing Li+ concentration, and is thus denoted as thecoordinated band, analogous to the coordinated C═O stretch which isapparent in the LiPF₆/carbonate electrolytes.

There are many spectral similarities in the C═O stretch region betweenthe amide and carbonate electrolytes; however, there are alsosubstantial differences. For instance, the spacing between the free andcoordinated bands was ˜32 cm⁻¹ for both linear and cyclic carbonates;conversely, the free and coordinated bands of the amide electrolytes aremuch closer together. Specifically, the spacing between the bands is ˜17cm⁻¹ for the cyclic amide and ˜25 cm⁻¹ for the linear amide.Furthermore, while the cyclic carbonate presented much broader FTIRbands than its linear analogue, the cyclic and linear amides show verysimilar bandwidths of ca. 25 cm⁻¹ full width at half maximum, FWHM.Lastly, the free carbonyl stretch band is nearly depleted at high Li⁺concentration, an effect which was not seen in the carbonateelectrolytes. The presence of two bands in the C═O stretch region of theFTIR for the amide electrolytes is a good indicator that the amidessolvate the lithium ion tetrahedrally, similar to the carbonatemolecules.

Though the amides appear to solvate Li⁺ tetrahedrally, there arespectral differences in the FTIR and 2DIR data which may indicatedifferences in the Li⁺ solvation structures. For instance in theconcentration dependent FTIR data, the free and coordinated bands aremuch closer together in frequency for the amide solvents than for thecarbonate solvents. It has been previously shown that as the interactionenergy increases, the separation of the free and coordinated band alsoincreases. Thus, this indicates that both of the amides have a weakerinteraction with the Li⁺ than the carbonate molecules, and furthermore,the cyclic amide has a weaker interaction with Li⁺ than its linearanalogue. This weaker interaction likely indicates that the solvatingamide molecules have a larger Li⁺—O distance on average.

The FTIR data showed that the free carbonyl band is almost completelydepleted at high lithium ion concentration, X(Li⁺)˜0.24 for the DMI andTMU electrolytes. This is also apparent in the fractional area of thecoordinated band with concentration as shown in FIG. 4. At highconcentration, the fractional area of the coordinated carbonyl stretchapproaches one for each of the amide electrolytes. At comparable Li⁺concentration, the coordinated band fractional area of the cyclic andlinear carbonates were reported at 0.9 and 0.8, respectively. Further,the fractional area of the coordinated bands of the amide electrolytesare higher at lithium salt concentrations of ˜1M than their carbonateanalogues. A high fractional area of the Li+-coordinated band waspreviously shown with DFT calculations to indicate Li⁺—O═C angles closerto 180°. Thus, the FTIR data indicates that the amides form a moretetrahedral (with respect to the Li⁺—O═C angle) solvation complex thanthe carbonates. The coordinated band fractional areas were modeled withan exponential growth function of the form: y0(1−e^((−X(Li) ⁺ ^()/C))),where C is the characteristic growth of the coordinated fractional areawith Li⁺ concentration. Both the cyclic and linear amides give similarcharacteristic growths of the coordinated band as their linearcounterparts. The ability of the free carbonyl stretch to be depleted athigh Li⁺ concentration also may indicate that the amide solvents do notsolvate the TFSI⁻ anion as well as the carbonate solvents, since themolecules participating in the solvation shell of the anion shouldappear in the free band. Alternatively, this could indicate that LiTFSIis able to participate in the solvation complex of Li⁺ at highconcentration. Further, an electrolyte was formed with the lithium salt,LiClO₄, in a mixture of butylene carbonate and tetramethylurea (TMU),and it was seen from the FTIR that all of the carbonate moleculesremained free in solution and only the TMU interacted with Li⁺. Thus, itappears that the amides, TMU in particular, solvated Li⁺ as well andpossibly better than carbonate solvents.

Turning to FIG. 5, it appears that the amide solvents solvate the PF₆ ⁻anion, the anion of another commonly used lithium salt, better thanlinear carbonate molecules. It has been shown previously that the P—Fstretch modes of the PF₆ ⁻ anion show a single band around 840 cm⁻¹ whenthe PF₆ ⁻ anion is well-solvated, and three bands at 830, 840, and 865cm⁻¹ when the PF₆— anion is not well-solvated and thus forms solventseparated ion pairs with the Li⁺. Using LiPF₆ as the lithium salt,electrolytes were prepared with two carbonates: dimethyl carbonate,which is linear, and propylene carbonate, which is cyclic, and with twoamides: TMU, which is linear, and1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMP), which iscyclic. All linear carbonates, though only dimethyl carbonate is shown,present three P-F stretching bands, indicating that the linearcarbonates do not solvate the PF₆ ⁻ anion well. However, both the linearand cyclic amide show a single P—F stretching band, indicating that theamides are good solvents both for the cation (Li⁺) and the anion (PF₆ ⁻,in this case).

Another noticeable spectral feature in the FTIR data (FIG. 3) for theamide electrolytes is that the FHWM of the coordinated bands arecomparable for the cyclic and linear solvents at any given LiTFSIconcentration over the range measured. This is in stark contrast withthe carbonates, where the FWHM of the coordinated band of the cycliccarbonates was nearly double that of the linear carbonates at any givenLi⁺ concentration. Furthermore, the FWHM of the coordinated bands inboth of the amide-electrolytes is very similar to that of the linearcarbonates. It has been suggested from NMR and infrared studies that asecond solvation shell of cyclic carbonates may be capable ofintercalating into the primary solvation complex around Li⁺, while thereis no evidence of the linear carbonate doing this. This may occurbecause of the size and rigidity of the cyclic carbonates. Conversely,as shown in FIG. 1, both the linear and cyclic amides have additionalmethyl groups flanking the carbonyl group and increasing the size of theamides relative to their carbonate analogues. These features may inhibitthe amides from the ability to intercalate into the solvation shell asevidenced by the narrower bandwidth of the coordinated carbonyl stretch.

An important characteristic of a battery electrolyte is itselectrochemical window, or the voltage range over which the electrolyteis stable. The electrochemical windows of the electrolytes formed from1M LiTFSI in DMI and 1M LiTFSI in TMU were determined from cyclicvoltammetry experiments to be 4.1 V and 4.3 V, respectively, as shown inFIG. 4. Currently used lithium-ion electrolytes have an electrochemicalwindow of approximately 4.5 V, meaning that the amide-based electrolytestested in this example are capable of operating under similar conditionsas the electrolyte which are currently used.

Two other common metrics for the physical characterization ofelectrolytes are conductivity, the ability of charge to move through thesolution, and viscosity. Shown in FIGS. 5 and 6 are the measuredroom-temperature conductivities and viscosities for the two amide-basedelectrolytes. Both amide-based electrolytes were measured to haveconductivities >3 mS/cm. While these conductivities are still lowcompared to the ˜10 mS/cm achieved by the current carbonate-basedelectrolytes, conductivity has been shown to be sensitive to the saltcomposition, solvent composition, salt concentration, and temperature.For instance, by replacing the LiTFSI salt with LiPF₆, the conductivityin TMU was increased to 5.6 mS/cm. Thus, it is believed that theconductivity of amide-based electrolytes may be improved by finding theoptimum electrolyte composition.

The experiments show that the amide-based electrolytes present atetrahedral solvation complex around Li⁺, which is less rigid than thatpresented by cyclic carbonates and of similar rigidity to that presentedby the electrolyte formed by linear carbonates. In short, theLi⁺-amide-based electrolytes provided conductivities less than thoseobserved for commercial lithium ion electrolytes, i.e., 10.4 mS/cm.However, this tradeoff in conductivity is balanced with a much largerflash point and, thus, a much larger temperature safe operating zone.Furthermore, it has also been shown that the conductivity of theelectrolyte can be increased by a modification of the salt—for example,when the salt is changed from LiTFSI to LiPF₆. Thus, it is believed thatthe conductivity can be recuperated by modifying the salts. Furthermore,the electrochemical windows of the amide-based electrolytes is verysimilar to that of the currently used carbonate-based electrolytes,indicating that the amide-based electrolytes may serve as significantlysafer alternatives to the carbonate-based electrolytes.

The subject matter of the present invention is described withspecificity herein to meet statutory requirements. However, thedescription itself is not intended to necessarily limit the scope ofclaims. Rather, the claimed subject matter might be embodied in otherways to include different steps or combinations of steps similar to theones described in this document, in conjunction with other present orfuture technologies. References are made in the text to exemplaryembodiments of an amide based electrolyte battery, only some of whichare described herein. It should be understood that no limitations on thescope of the invention are intended by describing these exemplaryembodiments. One of ordinary skill in the art will readily appreciatethat alternate but functionally equivalent components, materials,designs, and equipment may be used. The inclusion of additional elementsmay be deemed readily apparent and obvious to one of ordinary skill inthe art. Specific elements disclosed herein are not to be interpreted aslimiting, but rather as a basis for the claims and as a representativebasis for teaching one of ordinary skill in the art to employ thepresent invention.

Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized should be or are in any single embodiment. Rather,language referring to the features and advantages is understood to meanthat a specific feature, advantage, or characteristic described inconnection with an embodiment is included in at least one embodiment.Thus, discussion of the features and advantages, and similar language,throughout this specification may, but do not necessarily, refer to thesame embodiment.

Furthermore, the described features, advantages, and characteristics maybe combined in any suitable manner in one or more embodiments. Oneskilled in the relevant art will recognize that the Amide-BasedElectrolyte Battery may be practiced without one or more of the specificfeatures or advantages of a particular embodiment. In other instances,additional features and advantages may be recognized in certainembodiments that may not be present in all embodiments.

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment. Thus, appearances of the phrases“in one embodiment,” “in an embodiment,” and similar language throughoutthis specification may, but do not necessarily, all refer to the sameembodiment.

It should be understood that the drawings are not necessarily to scale;instead, emphasis has been placed upon illustrating the principles ofthe invention. In addition, in the embodiments depicted herein, likereference numerals in the various drawings refer to identical or nearidentical structural elements.

Moreover, the terms “substantially” or “approximately” as used hereinmay be applied to modify any quantitative representation that couldpermissibly vary without resulting in a change to the basic function towhich it is related.

What is claimed is:
 1. An energy storage apparatus comprising: a batteryhousing; a negative electrode; a positive electrode; and an electrolytecomprising a salt that has been dissolved in an amide-based solvent,wherein the amide-based solvent is selected from one or more of thefollowing: 2-imidazolidinone;1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone;1,3-dihydro-2H-imidazol-2-one; 1-isopropyl-2-imidazolidinone; and1-methyl-2-imidazolidinone.
 2. The energy storage apparatus of claim 1,wherein the amide-based solvent comprises: a solvent comprising at leastone of methyl urea; 2-imidazolidinone;1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone;1,3-dihydro-2H-imidazol-2-one; 1-isopropyl-2-imidazolidinone; and1-methyl-2-imidazolidinone; and a solvent comprising at least oneorganic carbonate, wherein the one or more of N,N′-dimethyl urea,1,1-dimethyl urea, N-ethyl urea, 1,3-diethyl urea, 1,1-diethyl urea,N-[(trimethylsilyl)methyl] urea and1[3-(trimethoxysilyl)propyl]ureaselected has a higher flash-point than the one or more selected organiccarbonates.
 3. The energy storage apparatus of claim 1, wherein theapparatus is a lithium ion battery.
 4. The energy storage apparatus ofclaim 1, wherein apparatus is a lithium ion battery and the positiveelectrode comprises lithium.
 5. The energy storage apparatus of claim 1,wherein the apparatus is a lithium ion battery and the salt compriseslithium.
 6. The energy storage apparatus of claim 1 wherein theapparatus is a lithium ion battery and at least one of the positiveelectrode or the salt comprises lithium.